(CNN)President Donald Trump is the most famous bone spur victim in the world. I should know -- I saw his foot when I questioned him about the real reason for his deferment from the military during the Vietnam War. And that's why I have little doubt about whom Sen. John McCain was talking in his C-SPAN interview on Sunday.

And just why was McCain speaking out? Perhaps it has something to do with Trump's 2016 attack on McCain. "I like people who weren't captured," he said as he questioned McCain's standing as a war hero. Or maybe it was Trump's mishandling of contacts with the families of fallen soldiers or his threatening declaration, aimed at McCain, that, "I'm being very, very nice. But at some point I fight back, and it won't be pretty."

Once again, the President is engaged in a controversy beneath the dignity of his office. This episode is a little different because he's warring with a respected senator from his own party who has received the sympathy and admiration of the nation as he deals with a virulent form of brain cancer. But as with so many of his previous bouts of nonsense -- remember when he said President Obama "tapped" his phones? -- this conflict is one he has manufactured himself with ridiculous statements and, to borrow a term from McCain, "half-baked" recollections.

The bone spur story is something I discussed with Trump when I interviewed him for my biography of him, "Never Enough: Donald Trump and the Pursuit of Success," published in 2015. As with so many things we discussed in many hours of interviews, this one required a painstaking dissection of Trump's record in order to sort out. In the past, he had noted student deferments and a high number in the draft lottery, which was based on the birthdays of eligible men, to explain why he never served. He even talked about watching the lottery on TV while he was at the University of Pennsylvania to see how he fared.

Having learned to check and double-check everything Donald Trump claimed, I had done the research to determine that the lottery had nothing to do with his Vietnam draft status. According to his Selective Service record, he had been granted a medical deferment a year before his lottery number was assigned in December 1969 (though his precise diagnosis was not included).

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Few issues mattered more to Americans of the Vietnam era than who served and who didn't. Theoretically, every male of a certain age was subject to being drafted. However, there were exemptions granted to college students and those who could obtain the right kind of letter from a doctor -- and those exemptions contributed to a great divide between those who fought and those who stayed home. Add the anti-war sentiments that grew into campus protests and the death toll that mounted among those who shipped out, and the division became even more bitter.

In my interview with Trump, I broached the subject with care, but insisted on getting to the bottom of things:

Interviewer: I want to get this resolved, and I want you to help me figure it out and deal with it.

Donald Trump: Go ahead.

Interviewer: The draft thing.

Donald Trump: OK.

Interviewer: Your [lottery] number was very high.

Donald Trump: Right.

Interviewer: But that was 18 months after you became eligible [for deferment].

Donald Trump: I don't know. I'm going to have to get the facts because they have a whole thing written down. I will get you facts, but it's very, very easy. Number one, the good news is I had a very, very high draft number.

Donald Trump: Because I have it written down. I'd rather give you exact dates and details. I have it written downstairs.

Interviewer: I don't do the gotcha thing. I come to you.

Donald Trump: Right, I understand that.

Interviewer: So you did have a medical deferment.

Donald Trump: Feet.

Interviewer: What was it for?

Donald Trump: The medical deferment is feet.

Interviewer: So what was going on with your feet?

Donald Trump: I have spurs on the back of my feet, which at the time, prevented me from walking long distances.

Interviewer: So you couldn't march?

Donald Trump: It would have been very difficult to march long distances. Very healthy, but in the back, in fact it is here. You can see it on both feet. I have spurs.

As he explained his condition, the man who would become president took off his shoes and pointed at his right heel. He asked me to take a look for bumps, which I didn't actually see, but I gave him the benefit of the doubt. He blamed misunderstanding about the issue on his off-the-cuff style, which left people with the impression that he was the lucky beneficiary of the lottery picks and not someone who had sought to be excused for a minor medical problem.

In our interview, I thought matters were settled and that he had acknowledged the truth, but very little stays settled where Trump is concerned. After our interview, and just a few weeks before my book was published, then-candidate Trump revisited the topic and reverted to his old story, "I had a minor medical deferment for feet, for a bone spur of the foot, which was minor," he said. "I was fortunate, in a sense, because I was not a believer in the Vietnam War. ... But I was entered into the draft and I got a very, very high draft number."

Why did Trump do this? I don't think it's because his story is better or less complicated than the actual facts. I think it's because going with the facts would require him to back down from his original tale about how he was saved by the lottery number. And, as everyone knows, the President never backs down. This is why he went after John McCain in the first place, and continues the fight even though he looks pathetic doing it.